The support of the hanging wall in mining stopes is one of the most basic requirements in mining. Dependent on the type and quality of rock being supported, the depth of mining, the prevalent field stresses, seismicity, stoping width and a number of other factors, stope support can vary across a vast range of materials, configurations and systems. These include, among others, gum poles, timber and composite packs, steel props, back-fill paddocks, unmined ore pillars, hanging wall rock anchors and any combination of the above.
Grout packs are among the increasingly utilized combination support products consisting essentially of a support column formed by a geotextile bag holding cured cemented back-fill or a similar cured cementious grout that is resistant to compression. The geotextile bag is usually protected and supported against lateral dilation of the pack under load by a wire or polymer mesh, as well as a set of additional wire or polymer rings surrounding the bag and mesh horizontally. The grout column is usually combined with timber poles that are required to suspend the bag, net and ring assembly prior to filling with grout.
For the purpose of this background discussion, the structural and support contribution of the timber poles to the behavior and performance of the grout pack shall be disregarded.
Under vertical (axial) load the grout column reduces in length and dilates laterally according to the Poisson's ratio of the grout material. Besides the cohesion of the cemented material, the geotextile bag (a), the surrounding mesh (b), as well as the restraining rings (c) all contribute in some measure to the support resistance of the pack in that they restrain the lateral dilation of the grout column.
(a) The geotextile material is usually woven or knitted from low tenacity polymer fibres and offers little lateral confinement as it stretches easily under load. Although it will provide some useful confinement, its primary function is to provide suitable containment for the grout slurry with optimal drainage and filtering properties.
(b) The secondary mesh basically forms a support structure for the geotextile material, preventing excessive bulging (with the associated increased solids losses through the enlarged pores) under hydrostatic loading of the uncured grout slurry. To add some degree of yieldability to the cured pack, the netting wires (or fibres) are usually oriented at 45° to the axis of the pack allowing the mesh to stretch in the horizontal direction, providing some additional lateral confinement to the pack.
(c) The lateral restraining rings are the major structural confinement of the pack and their strengths contribute directly and significantly to the support resistance of the pack. In conventional grout packs the performance of these rings is essentially dependent on their material properties, characterized primarily by their tensile strength and elongation. Invariably there is a trade-off in terms of these properties in that higher tensile strength generally goes with lower elongation and vice versa.
In stope support the stiffness of a support unit has to be carefully considered, however, as stronger and stiffer is not necessarily better, particularly in seismic stress environments where, under dynamic loading, shear stresses in the hanging wall around a very stiff pack can exceed the strength of the rock resulting in hanging wall failure (“punching”). Under such conditions, a yielding support unit should be able to absorb large and/or sudden rock movement without losing its structural integrity. Similarly, high closure stopes also require yieldability to safely absorb the energy of the closing hanging wall.
In conventional grout packs, the width-to-height ratio of the grout columns is insufficient to generate their own cemented material confinement under compression and the simple tendon lateral restraining rings, as described in (c) above are, therefore, the only significant lateral confinement of these packs.
It is these rings that largely control the compression behaviour of the packs. At present, however, they do not permit adequate yielding of the packs from an unyielded initial condition to a fully yielded condition as they rely solely on material deformation to permit yielding. Yield is thus determined by the quality of the steel used for the elements. After expansion permitted by the material yield of the elements the elements break and expansion becomes uncontrolled.
In this specification, yield refers to two separate concepts:                a) yield or elongation as a material property is the deformation of a material (e.g., a metal) beyond its elastic limit; i.e., yield or elongation is irrecoverable plastic deformation;        b) yield as a structural property refers to the plastic deformation of a structure, e.g., a grout pack; an “unyielded condition” refers to the condition of the grout pack immediately after being filled and a “fully yielded condition” refers to the condition of the grout pack after being subjected to axial loading wherein the diameter thereof increases according to the Poisson's ration of the material of which the structure is composed.        